Conventional gas turbine engines include an annular combustor having a plurality of circumferentially spaced carburetors disposed in an annular dome at an upstream end thereof. The combustor is disposed radially inside an annular casing and is provided with hot compressor discharge air which, depending on particular engine designs, may be from about 1,000.degree. F. (538.degree. C.) to about 1,400.degree. F. (760.degree. C.). To channel fuel to the carburetors within the annular casing, conventional fuel stems extend to the carburetors from an annular manifold surrounding the casing. The fuel stems are exposed to the hot compressor discharge air and must be suitably constructed for channeling the fuel without adverse effect.
For example, conventional fuel manifolds and the fuel stems are made from stainless steel or Hastolly X to provide adequate strength for withstanding the fuel pressure therein. These metals are also preferred since they do not react chemically with the fuel at the relatively high temperatures in the environment of the combustor. Although these metals have these advantages, they also have the disadvantage of having relatively high thermal conductivity which allows heat to be transferred to the fuel flowing in the manifold and fuel stems, which will raise the temperature of the fuel unless suitable means are provided for protecting the fuel.
The fuel stems exposed to the compressor discharge air are typically designed to be cooled, for example by the fuel itself or by insulating air gaps, to ensure that the temperature of the fuel being channeled therethrough does not reach unacceptably high temperatures which would cause undesirable coking of the fuel inside the fuel channels. It is desirable to maintain the internal wetted wall temperature of the fuel tubes or conduits less than about 350.degree. F. (177.degree. C.) for preventing coking of the fuel therein.
In order to meet this requirement, conventional fuel manifolds are mounted outside the casing surrounding the combustor wherein the environment is substantially cooler than that inside the casing wherein the compressor discharge air flows. Since conventional fuel manifolds typically extend about 360.degree. around the engine centerline and the casing surrounding the combustor, they are relatively large diameter structures and therefore increase the weight and envelope of the engine. Although the fuel stems connecting the manifold to the fuel injectors in the carburetors extend through the casing to the combustor in the environment of the hot compressor discharge air, they typically have relatively small diameters and are relatively short, and therefore may be more effectively cooled by the fuel channeled therein, or by insulating air gaps therein.